The embryoid body derived from transplantable mouse teratocarcinoma resembles mouse embryos of 5 days gestation ontogenically and morphogenically. The embryoid body differentiated in vitro in the same media in which normal mouse embryos develop. The factors in fetal calf serum which mediate the growth and differentiation of embryoid bodies consists of at least two components: (1) a dialyzable small molecule and (2) a non-dialyzable macromolecule. A teratocarcinoma cell line derived from an embryoid body has been established in tissue culture. The tumorgenicity of each cell type is under study.